How next-generation nuclear reactors break out of the 20th-century blueprint

Welcome to MIT Technology Review Narrated. My name is Matt Honan. I'm our Editor-in-Chief. Every week, we'll bring you a fascinating, new, in-depth story from the leading edge of science and technology, covering topics like AI, biotech, climate, energy, robotics, and more. Here's this week's story. I hope you enjoy it. Casey Crownhart writes, How next-generation nuclear reactors break out of the 20th century blueprint. Commercial nuclear reactors all work pretty much the same way. Atoms of a radioactive material split, emitting neutrons. Those bump into other atoms, splitting them and causing them to emit more neutrons, which bump into other atoms, continuing the chain reaction. That reaction gives off heat, which can be used directly or help turn water into steam, which spins a turbine and produces electricity. Today, such reactors typically use the same fuel, uranium, and coolant, water, and are all roughly the same size, massive. For decades, these giants have streamed electrons into power grids around the world. Their popularity surged in recent years as worries about climate change and energy independence drowned out concerns about meltdowns and radioactive waste. The problem is, building nuclear power plants is expensive and slow. A new generation of nuclear power technology could reinvent what a reactor looks like, and how it works. Advocates hope that new tech can refresh the industry, and help replace fossil fuels without emitting greenhouse gases. Demand for electricity is swelling around the world, Rising temperatures and growing economies are bringing more air conditioners online. Efforts to modernise manufacturing and cut climate pollution are changing heavy industry. The AI boom is bringing more power data centres online Nuclear could help but only if new plants are safe reliable cheap and able to come online quickly Here's what that new generation might look like. Every nuclear power plant built today is basically bespoke, designed and built for a specific site. But small modular reactors, SMRs, could bring the assembly line to nuclear reactor development. By making projects smaller, companies could build more of them, and costs could come down as the process is standardised. If it works, SMRs could also mean new uses for nuclear. Military bases, isolated sites like mines, or remote communities that need power after a disaster could use mobile reactors, like one under development from US-based BWXT in partnership with the Department of Defence. Or industrial facilities that need heat for things like chemical manufacturing could install a small reactor, as one chemical plant plans to do in cooperation with the nuclear startup X-Energy. Two plants with SMRs are operational in China and Russia today, and other early units will likely follow their example and provide electricity to the grid. In China, the Linglong One demonstration project is under construction at a site where two large reactors are already operating. The SMR should come online by the end of the year. In the US, Kairos Power recently got regulatory approval to build Hermes II, a small demonstration reactor. It should be operating by 2030. One major question for smaller reactor designs is just how much an assembly line approach will actually help cut costs. While SMRs might not themselves be bespoke, they'll still be installed in different sites, and planning for the possibility of earthquakes, floods, hurricanes, or other site-specific conditions will still require some costly customisation. When it comes to uranium, the number that really matters is the concentration of uranium-235, the type that can sustain a chain reaction Most uranium is a heavier isotope U238 which can Naturally occurring uranium contains about 0 uranium so to be useful it needs to be enriched, concentrating that isotope. Material used for nuclear weapons is highly enriched, to U-235 concentrations over 90%. percent. Today's commercial nuclear reactors use a much less concentrated material for fuel, generally between 3% and 5% U235. But new reactors could bump that concentration up using a class of material called high-assay low-enriched uranium, HALU, which ranges from 5% to 20% U235, still well below weapons-level enrichment. That higher concentration means halo can sustain a chain reaction for much longer before the reactor needs refuelling. How much longer varies with concentration, higher enrichment, longer time between refuels. Those higher percentages also allow for alternative fuel architectures. Typical nuclear power plants today use fuel that's pressed into small pellets, which in turn are stacked inside large rods encased in zirconium cladding. But higher concentration uranium can be made into tristructural isotropic fuel, or TRISO. TRISO uses tiny kernels of uranium, less than a millimetre across, coated in layers of carbon and ceramic, that contain the radioactive material and any products from the fission reactions. Manufacturers embed these particles in cylindrical or spherical pellets of graphite. The actual fuel makes up a relatively small proportion of these pellets' volume, which is why using higher enriched material is important. The pellets are a built-in safety mechanism, a containment system that can resist corrosion and survive neutron irradiation and temperatures over 3,200 degrees Fahrenheit, 1,800 degrees Celsius. Fission reactions happen safely inside all these protective layers which are designed to let heat seep out to be ferried away by the coolant unused The coolant in a reactor controls temperature and ferries heat from the core to wherever it used to make steam, which can then generate electricity. Most reactors use water for this job, keeping it under super high pressures so it remains liquid as it circulates. But new companies are reinventing that process with other materials, gas, liquid metal or molten salt. These reactors can run their coolant loops much hotter than is possible with water, upwards of 500 degrees C, as opposed to a maximum of around 300 degrees C. That's helpful because it's easier to move heat around at high temperatures, and hotter stuff produces steam more efficiently. Alternative coolants can also help with safety. A water coolant loop runs at over 100 times standard atmospheric pressure. Maintaining containment is complicated but vital. A leak that allows coolant to escape could cause the reactor to melt down. Metal and salt coolants, on the other hand, remain liquid at high temperatures but more manageable pressures, closer to one atmosphere. So those next-generation designs don't need reinforced high-pressure containment equipment. These new coolants certainly introduce their own complications, though. Molten salt can be corrosive in the presence of oxygen, for example, so builders have to carefully choose the materials used to build the cooling system. And since sodium metal can explode when it contacts water, containment is key with designs that rely on it. Ultimately, reactors that use alternative coolants or new fuels will need to show not only that they can generate power, but also that they're robust enough to operate safely and economically for decades. You were listening to MIT Technology Review, where Casey Crownhart writes how next-generation nuclear reactors break out of the 20th century blueprint. This article was published on the 12th of January 2026 and was read by Jane Wing for NOAA.